1. Field of the Invention
The present invention relates to a method of depositing a kesterite film. More particularly, the present invention relates to a method of depositing a kesterite film from an aqueous dispersion.
2. Description of Related Art
Large-scale production of photovoltaic devices requires high-throughput technologies and abundant environmentally friendly materials. Thin-film chalcogenide-based solar cells provide a promising pathway to cost parity between photovoltaic and conventional energy sources.
Currently, only Cu(In,Ga)(S,Se)2 and CdTe technologies have reached commercial production and offer over 10 percent power conversion efficiency. These technologies generally employ (i) indium and tellurium, which are relatively rare elements in the earth's crust, or (ii) cadmium, which is a highly toxic heavy metal.
Copper-zinc-tin-chalcogenide kesterites have been investigated as potential alternatives because they are based on readily available and lower cost elements. However, photovoltaic cells with kesterites, even when produced using high cost vacuum-based methods, have so far achieved at best only <6.7 percent efficiencies, see Katagiri, H. et al. Development of CZTS-based thin film solar cells; Thin Solid Films 517, 2455-2460 (2009).
K. Tanaka, M. Oonuki, N. Moritake, H. Uchiki, Solar Energy Mater. Sol. Cells 2009, 93, 583-587 describe a solution-based approach for an indium-free material which produced a photovoltaic device with efficiency of only 1%.
T. Todorov, M. Kita, J. Garda, P. Escribano, Thin Solid Films 2009, 517, 2541-2544 describe a deposition approach based on quaternary Cu—Zn—Sn—S precursors formed by reacting metal acetates and chlorides with elemental sulfur in ethylene glycol at 170° C.
Guo et al, J. Amer. Chem. Soc., 2009, 131, 11672-11673 have reported films deposited by a similar approach, subsequently subjected to selenization treatment. They have also reported that devices based on the Cu2ZnSnSySe1−y films yield efficiencies of 0.74%, a level that is lower than the above solution approach for Cu2ZnSnS4.
The commonly owned and previously incorporated U.S. application Ser. No. 12/606,210 and a publication by T. Todorov, K. Reuter, D. B. Mitzi, Advanced Materials, (2010) Vol. 22, pages 1-4, describe a hydrazine-based deposition approach of depositing homogeneous chalcogenide layers from mixed slurries containing both dissolved and solid metal chalcogenide species dispersions of metal chalcogenides in systems that do not require organic binders. Upon anneal the particle-based precursors readily react with the solution component and form large-grained films with good electrical characteristics.
However, the use and transportation of pure hydrazine is regulated by the federal government because improper handling is associated with risk of ignition and pure hydrazine is used to propel rockets and missiles. Particularly hazardous are pure hydrazine vapors that, in the absence of a diluent gas (such as ammonia, water vapor or nitrogen), may explode if ignited, in contrast to other inflammable solvents that require oxygen to form explosive mixtures. Transportation and use of pure hydrazine in manufacturing therefore requires rigorous and expensive handling protocols in order to assure safe large-scale photovoltaic manufacturing based on prior-art methods.
Another processing challenge associated with previous methods is the exceptional activity of pure hydrazine as a solvent, making it incompatible with numerous materials typically used in processing equipment, such as a variety of plastics and metals. In addition, control of the ratio between the dissolved and solid component in pure hydrazine is difficult to achieve in order to optimize rheological and coating characteristics of the obtained inks.